Derive deduplicated error issues from traces and logs (Phase 2 backend)

- Add internal/issues.Tracker: fingerprints (kind, service, title) into
  deduplicated Issues with count/firstSeen/lastSeen/affected services and
  bounded sample occurrences, evicting the least-recently-seen issue once
  a cap is reached.
- Add internal/collector/issue_extract.go to pull issues out of ingested
  OTLP data: spans with an error status (preferring exception.type/message
  from an "exception" event, falling back to the span status message), and
  logs at ERROR/FATAL severity (by severity number or text).
- Wire extraction into Collector.ConsumeTraces/ConsumeLogs and expose
  GET /api/issues and GET /api/issues/{id}.
- Fix a pre-existing data race in the collector test's mockStore (unsynced
  access to entries while addToHistory appends asynchronously), found via
  `go test -race` while adding the new issue-tracking tests.
- Update docs/PLAN.md, README.md and agents_example.md for the new API.

The UI "Issues" tab itself lives in the separate frontend project that
builds this server's dist/ bundle; this commit only covers the backend.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -106,6 +106,16 @@ gRPC services for traces, metrics, and logs using standard OTLP protobuf definit
 All endpoints above search the full persistent store (when `OTELLOG_PATH`
 storage is enabled), not just the in-memory ring buffer.
 
+### Issues API Endpoints (Port 8080)
+
+Errors seen in traces (spans with an error status) and logs (severity
+`ERROR`/`FATAL`) are deduplicated into "issues" as they're ingested.
+
+- `GET /api/issues` - List deduplicated issues, most recently seen first.
+  Supports `limit`.
+- `GET /api/issues/{id}` - Fetch a single issue, including up to 5 recent
+  sample occurrences (timestamp, trace/span id, message).
+
 ### Web Interface
 
 - `GET /` - Serves the HTML visualization interface